Atomic and electronic structures of superconductingBaFe2As2/SrTiO3superlattices

Abstract

The nanoscale structural, chemical, and electronic properties of artificial engineered superlattice thin films consisting of superconducting Co-doped BaFe2As2 (Ba-122) and nonsuperconducting SrTi03 (STO) layers are determined by using atomically resolved scanning transmission electron microscopy and electron energy loss spectroscopy. The bonding of Ba-122/STO occurring between As (Ba) and SrO (Ti02) terminated layers has been identified. The thin STO (3 unit ceil) insertion layers are a mixture of cations (Ba, Sr, Fe, and Ti) and rich in oxygen vacancies and the Ba-122 layers (10 unit cell) are free of vertical second phases. Our results explain why these superlattices show anisotropic transport response to an external magnetic field, i.e., strong ab-axis pinning (enhancing critical current density) and no c-axis pinning, which is opposite to single layer Ba-122 thin films. These findings reveal physical and chemical properties of superconducting/nonsuperconducting heterostructures and provide important insights into engineering of superconducting devices.High-quality epitaxial heterostructures consisting of su perconducting and nonsuperconducting layers have been attracting extensive research interest because their novel properties make them very attractive candidates for a variety of applications, such as Josephson junctions, superconducting quantum interference devices (SQUIDs), and other field-effect devices [1][2][3][4][5][6][7]. Epitaxial pnictide thin films, however, have so far been hard to produce due to the volatile elements in the ironbased superconductor phase and therefore it is difficult to con trol the stoichiometry of the deposited films [8,9]. By employ ing epitaxial SrTi03 (STO) templates, we were able to grow high-quality epitaxial Co-doped BaFe2As2 (Ba-122) thin films on (La,Sr)(Al,Ta)03 (LSAT) substrates [10][11][12]. Recently, we reported the growth and properties of artificially engineered superlattices of Co-doped BaFe2As2/SrTi0 3 (Ba-122/STO) and Co-doped BaFe2 As2/undoped BaFe2 As2 [12], The former was possible due to their similar structures of which bulk Ba-122 has a tetragonal structure with lattice parameter a = 3.962 A, c = 13.017 A [13,14], while cubic STO has a lattice parameter with a -3.905 A. As a result of small misfit (1.4%) at the interface, the films showed excellent epitaxial growth with the use of the oxide templates.Previously we reported that single layer Ba-122 thin films showed strong c-axis pinning effect which enhances the 'Present address: PACS number(s): 68.37.-d, 73.21.-b, 74.25.-q in-field performance of the critical current density [10,11].High-resolution TEM results indicated that the presence of the c-axis aligned self-assembled oxide nanopillars in Ba-122 thin film [ 15] accounts for the c-axis pinning effect. In contrast, superlattice structure of Ba-122/STO shows very strong abaxis pinning rather than c-axis pinning. The different transport properties in these thin film devices essentially originate from the nature of defects, i.e., heterointerfaces and se...

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